vp9_thread_test: quiet -Wmissing-prototypes

BUG=webm:1584

Change-Id: Ie514f6630acfb018a3ac4a05758c8b4119ae28fa
diff --git a/test/vp9_thread_test.cc b/test/vp9_thread_test.cc
index 8eff3f1..31b6fe5 100644
--- a/test/vp9_thread_test.cc
+++ b/test/vp9_thread_test.cc
@@ -196,6 +196,7 @@
 // Note any worker that requires synchronization between other workers will
 // hang.
 namespace impl {
+namespace {
 
 void Init(VPxWorker *const worker) { memset(worker, 0, sizeof(*worker)); }
 int Reset(VPxWorker *const /*worker*/) { return 1; }
@@ -208,6 +209,7 @@
 void Launch(VPxWorker *const worker) { Execute(worker); }
 void End(VPxWorker *const /*worker*/) {}
 
+}  // namespace
 }  // namespace impl
 
 TEST(VPxWorkerThreadTest, TestSerialInterface) {